In a 9.3 m high and 0.10 m i gas-solids downflow fluidized bed (downer), the radial distributions of the local solids holdups and the pressure gradients along the downer column were measured to characterize the local and overall gas-solid flow structure, using a fiber optical solids concentration probe and a series of pressure transducers. Results show that the radial solids holdup distribution in a downer reactor is much more uniform than that in a riser. Although the operating conditions do largely affect the hydrodynamics in the downer reactor, many common similarities still exist in the gas-solids flow patterns with the variation of the operating conditions. For most of the conditions tested, the radial profile of solids holdup reaches its fully developed state within 3-8 m from the top entrance. On the other hand, particles are fully accelerated within 1-4 m. However, under certain conditions, the development section can extend to as long as the downer itself.